1. Field of the Invention
Dye sublimation thermal transfer sheets comprising a base material sheet formed of a polyester film or the like and a sublimable dye-containing thermally sublimable colorant layer provided on one surface of the base material sheet and heat fusion thermal transfer sheets having the same construction as the dye sublimation thermal transfer sheets except that a fusion-transferable colorant layer comprising a thermally fusible composition containing a colorant is provided instead of the thermally sublimable colorant layer are known as thermal transfer sheets for image formation using thermal transfer. In these thermal transfer sheets, it is common practice to provide a heat-resistant slipping layer on a surface of the base material sheet remote from the colorant layer or to provide a primer layer between the base material sheet and the heat-resistant slipping layer, from the viewpoint of preventing fusion between a base material sheet and a thermal head.
An increase in printing speed in recent printers has led to a tendency toward a more and more increase in heat energy emitted from the thermal head, leading to problems derived from fusion between the heat-resistant slipping layer and the thermal head, for example, sticking, print cockles, and ribbon breakage. An attempt to impart further improved heat resistance to the heat-resistant slipping layer has been made in order to realize high speed printing in printers. However, it has been found that, when the conventional primer layer is used, the primer layer is softened by heat energy, resulting in print defects as a result of flow of the heat-resistant slipping layer, making it impossible to satisfactorily develop properties of the heat-resistant slipping layer.
2. Description of Related Art
For example, Japanese Patent Application Laid-Open No. 1653/2001 (patent document 1) discloses a thermal transfer sheet comprising a primer layer containing a sulfonated polyaniline as an antistatic agent and a resin having given viscosity and elasticity as a primer component. The claimed advantage of the thermal transfer sheet is that cockling of the thermal transfer sheet caused by heat damage of the primer layer during printing can be prevented by maintaining a high viscoelasticity of the primer layer under elevated temperature conditions.
In the thermal transfer sheet described in patent document 1, however, it is difficult to say that the thermal transfer sheet can satisfactorily withstand heat energy that is emitted from the thermal head and has been increased due to an increase in printing speed of recent printers. Accordingly, the development of thermal transfer sheets having higher heat resistance has been desired.